1. Field of the Invention
This invention relates to operating mode and parameter changes in artificial implantable medical device, e.g. cardiac pacemakers, in response to and to indicate depletion of the device's sensing power source.
2. Description of the Prior Art
The implantable cardiac pacemaker, shown in U.S. Pat. No. 3,057,356 and subsequent patents permits innocuous, painless, long-term cardiac stimulation at low power levels by utilizing a small completely implanted transistorized and battery operated pulse generator connected via a flexible lead bearing an electrode directly in contact with cardiac tissue. Most pulse generators consist of a stimulating circuit and a sensing circuit both of which draw current from the battery. In the presence of complete heart block, an asynchronous pulse generator with only a stimulating circuit may be used, however, in most instances, noncompetitive triggered or inhibited pulse generators having the sensing circuit are preferred and dominate the pacemaker market. The demand, synchronous or triggered pulse generators are especially useful in patients with spontaneous cardiac activity because of their ability to sense intrinsic cardiac rhythm (atrial or ventricular depending on variety and electrode position), and to alter the pacemaker output accordingly. Such pacemakers are shown for example, in U.S. Pat. Nos. 3,253,596 (P-wave synchronous), 3,478,746 (ventricular inhibited) and are described in the pacing literature.
More recently, attention has been paid to the physiological aspects of cardiac pacing therapy and particularly to pacing systems for maintaining synchronous atrial and ventricular depolarization of the heart. In early atrial synchronized (or A-V synchronous) pacing, atrial depolarization is sensed through one electrode, and after an appropriate delay the ventricle is paced through a different electrode, thereby restoring the normal sequence of atrial and ventricular contraction and allowing the pacer to respond to physiologic needs by increasing its rate. Below a predetermined minimal atrial rate, however, the pacemaker reverts to its basic ventricular pacing rate. In atrial synchronous, ventricular inhibited pacers of the type described in U.S. Pat. Nos. 4,059,116 and 3,648,707, the ventricular depolarizations are also sensed and inhibit or reset the timing of the ventricular stimulating pulse generator.
A more complex method of restoring synchrony is by the atrial ventricular sequential pacing of the type described in U.S. Pat. No. 3,595,242 and subsequent patents which possess atrial and ventricular pulse generators and associated electrodes and a ventricular sensing circuit. In atrial ventricular sequential pacing, the atria and ventricles are paced in proper sequence, the atrial and ventricular pulse generator timing circuits being reset on sensing spontaneous ventricular activity.
Finally, U.S. Pat. No. 4,312,355, by Hermann D. Funke, (assigned to a subsidiary company of the assignee of the present invention), discloses a pacemaker which, if required, may stimulate the atrium and/or ventricle on demand and which is able to maintain synchrony as the sensed atrial rate increases. A pacemaker of this type is capable of distinguishing between bradycardia and normal heart function and to provide atrial and/or ventricular pacing in the following modes: inhibited in the case where the atrium and ventricle beat at a sufficient rate; atrial demand in instances where the atrium is beating at an insufficient rate and must be stimulated whereas the ventricle properly follows; atrial synchronous when the atrium depolarizes at a sufficient rate but the ventricle does not follow within a prescribed A-V interval; and demand in both chambers when neither the atrium and the ventricle spontaneously depolarize at the desired rate.
All of the demand pacemakers of the types described above comprise ventricular and/or atrial timing circuits which may be a simple oscillator of the early designs or the complex, programmable, digital timing circuit of the type disclosed, for example, in the commonly assigned U.S. Pat. No. 4,230,120, an analog sense amplifier circuit of the type disclosed, for example, in the U.S. Pat. No. 4,266,551, by Marc T. Stein, and an analog output circuit of the type disclosed, for example, in the copending U.S. Ser. No. 184,777 filed Sept. 8, 1980 in the name of David L. Thompson, all assigned to the assignee of the present invention. The inputs of the respective sense amplifiers and the output capacitances of the output circuits are commonly coupled to the respective atrial or ventricular sense amplifiers and through pacing leads to the electrodes coupled to the patient's heart.
Such medical devices as the demand pacemakers of the types described above are powered by depletable power sources or batteries which deplete from a beginning of life (BOL) to an end of life (EOL) voltage and current condition. The depletion reduces the current and voltage available to power the various components of the pulse generator and provide an adequate stimulation energy leading to either loss of capability of the device or capture of the patient's heart. To alert the patient or physician of the condition of the battery, prior art pulse generators have provide end-of-life indicator circuits, usually activated by a magnetically actuated reed switch, which cause the pulse generator to operate in an asynchronous mode at a rate which differs from the rate provided at BOL in proportion to the power source depletion or to a predetermined EOL rate upon depletion or to a predetermined EOL rate upon depletion to a selected EOL energy level. Such prior art pulse generators of the type described in the aforementioned U.S. Pat. No. 4,230,120 possess such EOL circuitry for indicating to the physician the state of depletion of the power source. In a multi-programmable multi-mode medical device of the type described herein, several modes and operating parameters of the device may be selectively programmed. In the atrial and ventricular pacemaker of the type described herein, the energy drain on the power source is accentuated by the complexity and number of components in the circuitry and by the A-V sequential pacing output pulses which over the same period of time, at full pacing, draw current more rapidly from the battery than a conventional single chamber demand pacemaker. These characteristics of the pulse generator in which the present invention is embodied require a more complex and graduated manner in which the power source depletion may be monitored under several operating conditions.